The present invention relates to a method and apparatus for monitoring synchronization between base stations in a telecommunications system and, more specifically, to a method and apparatus for detecting synchronization deviation between the base stations.
In a cellular telecommunication system such as cellular telephone system 100 illustrated in FIG. 1, there are a plurality of cells, for example, cells C0-C9. Typically, each of the cells C0-C9 has a single base station, for example, base stations B0-B9. In general, each base station B0-B9 transmits and receives communication signals, using a base transceiver station (BTS), to and from mobile units within the corresponding cell. Therefore, the system of FIG. 1 includes BTS0-BTS9 corresponding to base stations B0-B9.
When a mobile unit such as M4 moves from a cell, C0, covered by a first base station transceiver BTS0 towards another cell, C1, covered by a second base station transceiver BTS1, the responsibility for communication with the mobile unit M4 must be transferred from BTS0 to BTS1. Such transfer in conjunction with an established connection to the mobile station is usually called a xe2x80x9chandoverxe2x80x9d.
Handover requires careful system management to reduce the risk of the established connection being temporarily interrupted or completely lost during the handover. Such interruption and/or loss of service may result from, among other things, lack of synchronization between reference timing sources employed by the first base station and the second base station. Therefore, maintaining synchronization between base stations in cellular telecommunication system is extremely important.
The BTS uses an external or internal reference timing source to generate accurate radio carrier frequencies (i.e., such as one having an absolute fault of approximately 0.1 parts per million [ppm]) as well as to generate accurate timing of the bit stream transmitted over the radio interface. In instances where the BTS uses an external source as a timing reference synchronization, the clock source is located in the public switching telephone network (PSTN). The BTS synchronization is then taken from the PSTN via the mobile switching center (MSC), the base station controller (BSC) and transmission networks. Outside the United States, a common timing reference is used for each BTS in a network. If the BTS uses an internal clock source on the other hand, then this source is usually a high stability oscillator. In the United States, each BTS has a local (internal) source.
If the timing reference employed by a BTS is unreliable, a mobile station attempting to synchronize itself to the BTS may encounter difficulties. In extreme cases, the mobile unit may not be able to access the BTS. For example, referring again to FIG. 1, consider that mobile unit M4 has a call set-up using BTS0 corresponding to base station B0. BTS0 is synchronized to the PSTN and M4 is synchronized to BTS0. Mobile station M4 will typically be requested to measure and report signal strength associated with neighboring base stations"" transmission for the purpose of identifying when handover to another base station is desirable. As M4 moves towards BTS1, if the timing reference employed by BTS1 is not reliable and, therefore, BTS1 is transmitting information at a different frequency and/or with a different timing than that expected by M4, then M4 may not be able to read the base station identity code (BSIC) associated with BTS1. In such a situation, M4 will not report BTS1 as a handover candidate to BTS0 which is currently in communication with M4. If BTS1 is not synchronized with the PSTN, the call will drop after M4 enters an area where only BTS1 can provide communication service support.
Unreliable sources of synchronization can cause frequency drift of the base stations over a period of time. These sources are one reason that cause calls to be dropped, lost or interrupted. Other reasons may include unsynchronized clocks, faulty configuration of intermediate network modules such as DXXs, DXCs and BSCs as well as BTS faults and link breaks.
The synchronization problems usually come to the attention of system operators via, among other means, end user (i.e., subscriber) complaints of dropped calls. As a result, identifying dropped calls that result from lack of synchronization is a time consuming process. Identifying a synchronization problem from statistical data is also a very slow and unreliable process, for example, in cases where the mobile unit of a particular brand is unable to read a base station identity code (BSIC) from a neighboring cell. Furthermore, if the synchronization deviation is changing slowly, the detection of this problem becomes harder.
What is desired, therefore, is a method and apparatus for detecting synchronization deviation in a cellular telecommunications system.
It is an object of the present invention to overcome the deficiencies described above by providing a method for utilizing existing functionality of a mobile unit for monitoring synchronization deviation in a cellular telecommunications system.
According to exemplary embodiments of the present invention, a system for monitoring synchronization in a radio telecommunication system is disclosed which comprises a first and a second base station each having a transceiver associated therewith; at least one mobile station having a transceiver and communicating with the first base station prior to being handed over to the second base station; a processor, associated with the at least one mobile station for determining a time difference value between a signal received from the first base station and a signal received from the second base station, wherein the mobile station transmits the time difference value to the system; and a controller, within the system, for receiving said time difference value and determining whether the first and second base stations are synchronized based on the time difference value.
Other embodiments of the present invention disclose a method for maintaining synchronization in a radio telecommunication system having a plurality of base stations and a plurality of mobile stations. The method comprises the steps of monitoring synchronization deviation between at least one pair of base stations within the plurality of base stations, wherein a deviation in the synchronization is computed by a base station controller (BSC) based on values reported to the BSC by a mobile station being handed over from a first one of the pair of base stations to a second one of the pair of base stations and on a value reported to the BSC by a base station transceiver associated with the second of said base stations; and triggering an alarm if the monitoring indicates that a predetermined threshold for a deviation in the synchronization has been exceeded.